Granted, it would be really difficult/a pain in the butt to do it off the top of a semi-trailer, but this is a prime example why, after a significant snowfall, it's a good idea for us regular folks to take a couple of extra minutes to clear off the snow that builds up on the tops of our cars.

TL;DR - clear off the snow from the tops of your car so that while you're driving, the snow doesn't come off and potentially damage other cars that happen to be on the road with you.